Hydrotalcites are derivatives of brucite, a naturally-occurring, layered, magnesium hydroxide mineral. Synthetic hydrotalcites can be made by substituting a trivalent metal cation, such as aluminum, for some of the magnesium cations normally present in a layer. The magnesium cations can also be substituted by other divalent cations. This substitution will result in a net positive charge residing on the layer, which requires an intercalating anion to achieve a net neutral charge for the molecule. The following general formula has been derived for synthetic hydrotalcites:[M2+1-xM3+x(OH)2]x+[An−x/n·mH2O]x−wherein M2+ is magnesium and/or other divalent cation, M3+ is aluminum and/or other trivalent cation and An− is an anion. In addition to the anion, it will be noted that water is also a part of the lattice structure.
A group of hydrotalcites with a unique sheet-like morphology is described in U.S. Pat. No. 5,399,329, issued to Schutz, et al., and assigned to the assignee of the present invention. The entire contents of the Schutz '329 patent are incorporated herein by reference. The hydrotalcites of the Schutz '329 patent are comprised of anions derived from C1 to C4 saturated carboxylic acids. The general synthetic method of the Schutz '329 patent involves the reaction of an alumina source with a carboxylic acid in water followed by the reaction of the resulting mixture with a magnesium source. The approximate molar ratio of the reagents is as follows: 2 Mg:1 Al:1 anion; with the anion being the carboxylate of the acid used.
Although a hexagonal morphology is normally observed for non-carboxylate anion hydrotalcites, the carboxylate anion hydrotalcites of the Schutz '329 patent exhibit a unique morphology, termed therein “sheet-like”. The distance between the hydrotalcite layers, as measured by d spacing, depends on the size of the intercalating anion. For example, carboxylate hydrotalcites from the following anions produced by the method of the Schutz '329 patent have a d spacing of: formate 7.64 Å, acetate 12.3 Å, propionate 13.02 Å, and isobutyrate 15.15 Å.
In the Schutz '329 patent, sheet-like hydrotalcites are prepared in aqueous medium by reacting alumina with a carboxylic acid at about 60° C. for 30 minutes followed by the addition of magnesium oxide at a temperature of 95° C. for about 6 hours. The desired gel hydrotalcite is obtained upon drying the reaction product. Although the method of the Schutz '329 patent works rather well for most water-soluble carboxylic acids such as C1 to C4 carboxylic acids, it does not work well for those acids, which are water-insoluble. In fact, butyric acid, which is a C4 acid, has only limited success in the method of the Schutz '329 patent.
Hydrotalcites have many uses, including such applications as catalysts or catalyst precursors, ion exchangers, ion absorbers, ion-scavengers, and medical uses as antacids. Hydrotalcites are also used as nanocomposites in polymers to provide various property enhancements. Hybrid composites of polymer and other inorganic components such as clays and mica have been described in the prior art as having improved mechanical properties. The term nanocomposites reflects the dispersion of nano-scale particulates of the inorganic component of the hybrid in the polymer matrix.
In Japanese Patent Application 96-189168, assigned to Mitsui Petrochem Ind. Ltd., naturally-occurring hydrotalcites containing a carbonate anion are used in polypropylene synthesis, along with other additives, and are said to give good melt flow index, flexural modulus and Izod impact strength.
In EP 0,910,131, assigned to AtoChem, Fr., naturally-occurring hydrotalcites containing a carbonate anion are used in an ethylene-vinylacetate copolymer and are said to produce a film with good adhesion and barrier properties.
In Japanese Patent Application 86-296799, assigned to Du-Pont Mitsui Polychemicals Co., Ltd., naturally-occurring hydrotalcites containing a carbonate anion are used in linear, low density polyethylene and are said to produce a film which has thermal insulating properties and good tensile strength.
Most nanocomposite polymer applications use pillared clays and/or naturally-occurring hydrotalcites. Compounded compositions of nylon-6 and 5% clay nanocomposits have been shown to exhibit a 40% higher tensile strength, 68% greater tensile modulus, 60% higher flexural strength and a 126% flexural modulus (See, Int'l. SAMLE Symp. Exhib. 1998, 43:1053-1066). Nanocomposites are believed to disperse in the polymer in one of the following two ways:                1) in a disorderly fashion, such as by intercalation; or        2) by exfoliation, in which the nanolayers are regularly spaced in the polymer. Exfoliation is believed to lead to improved polymer properties.        
There are references in both the patent and scientific literature of various clays, which have been modified and combined with polar polymers such as polyamides to form nanocomposite materials.
However, the introduction of nanoparticles into nonpolar polymers such as polyolefins to form a nanocomposite is a much more difficult task due to incompatibility of the polar nano particles with the nonpolar polymer. This incompatibility often leads to non-uniform distribution of the inorganic component throughout the polymer, leading to less than optimum performance. Typically, this difficulty is overcome by combining the nonpolar polymer with a similar, but chemically modified polymer (e.g. polypropylene-g-MA), which contains polar functionality to act as a compatibilizer molecule. The polar functionality of the modified polypropylene is able to interact with the polar character of the nanoparticle, and the nonpolar portion of the modified polypropylene interacts with the polypropylene matrix. Presumably, the interaction between the two polar functionalities provides both exfoliation and compatiblization, thereby resulting in a nanocomposite with uniform distribution of the nanoparticles.
U.S. Pat. No. 5,973,053 describes a layered composite clay material wherein organic onium ions and primary and secondary organic “guest” molecules are introduced into the interlayer space to increase the interlayer distance. The introduction of the organic onium ion acts to increase the compatibility of the clay with polymer and facilitate the dispersion of the clay in the hybrid composite.
In “Factors Controlling Mechanical Properties of Clay Mineral/Polypropylene nanocomposites”, Journal of Materials Sciences 35 (2000) 1045-1050, Oya et al describe intercalating a clay with a polar monomer, diacetone acrylamide and maleic acid modified polypropylene as a compatibilizer. This organo-clay was then mixed with conventional polypropylene to prepare a nanocomposite. In “Poly(propylene)/organo-clay nanocomposite formation: Influence of compatibilizer functionality and organo-clay modification”, Macromolecular Material Engineering 275, 8-17 (2000), Reichert et al describe the use of alkyl amines as intercalating agents in silica clay with and without the use of maleic anhydride modified polypropylene.
A need exists in the art for new synthetic hydrotalcites made from organic anions longer than C4 and also those with functional groups including saturated carboxylates of C6, C8, C10 and C18 straight chain acids; aromatics such as benzoates, chlorobenzoates, naphthoates, and p-hydroxybenzoates; carboxylates of acrylic, methacrylic and vinylacetic acids; and mixtures of these organic anions. Such new synthetic hydrotalcites can find among their uses, that as nanocomposites in polymer applications, because these synthetic hydrotalcites are customizable according to the properties desired in the polymers made therefrom. A need also exists for modified hydrotalcites made from carboxylates of C2 and higher organic acids containing heteroatoms such as nitrogen, sulfur, phosphorous and halogens, which can be used in polymer nanocomposites and are more easily dispersed in a non-polar polymer without the necessity of using a compatibilizer.